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mysql中null值的排序问题分析
如下表t_user：
name age
zhangsan 1
lisi null
wangwu 2
执行一下sql：
sql代码
select * from t_user order by age;
name age
lisi null
zhangsan 1
wangwu 2
实际上我们是想将没有填写age的记录放在最后，我们可以
sql代码
select * from t_user order by age is null, age;
name age
zhangsan 1
wangwu 2
lisi null
为什么会这样？可以这样来理解：
sql代码
select * from t_user order by age is null, age;
等价于:
sql代码
select * from (select name, age, (age is null) as isnull from t_user) as foo order by isnull, age;
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